The instant application should be granted the priority dates of Dec. 15, 2005, the filing date of the corresponding German patent application 10 2005 059 971.0, as well as Dec. 14, 2006, the filing date of the International patent application PCT/EP2006/012087.
The present invention relates to an apparatus for mixing a fluid with a large volume gas stream (principal stream) flowing in a gas duct, especially for introducing a reducing agent into a flue gas that contains nitrogen oxides, the apparatus having at least one nozzle or metering lance having at least one nozzle for the supply of the fluid, the axis of which forms an angle with the direction of flow of the gas stream, and having at least one flat mixer element that is associated at a distance with the nozzle and that forms an angle with the direction of flow of the gas stream, whereby flow eddies form at the mixer elements and at least a portion of the fluid passes or enters into these flow eddies.
Such an apparatus is known from DE 37 23 618 C1, whereby the reducing agent is introduced in a gaseous state into the large gas stream (flue gas). The static mixer element is used to shorten the fundamentally very long mixing paths.
With SCR units, to reduce the NOx level (Selective Catalytic Reduction) of flue gases, for example of power plant furnaces, by means of reducing agent and catalyzer, it is customary, where the reducing agent is NH3, that it is stored in the form of pressure-condensed NH3 or of ammonium hydroxide (NH4OH), and pre-vaporized NH3 is sprayed into the flue gas stream via a carrier gas stream and is mixed with the flue gas stream. Where the reducing agent is urea, first an aqueous urea solution is produced that, after suitable processing, is then sprayed into the flue gas stream in gaseous form.
With the known apparatus, the mixer element is a square or rectangular plate that extends over the width of the flue gas duct. The metering lance with the nozzle is disposed essentially laterally of and parallel to that edge of the mixer plate that is disposed upstream relative to the direction of flow of the flue gas stream, and the nozzle stream of the gaseous reduction agent, in the form of NH3-carrier gas mixture, is sprayed laterally upon the back side of the mixer element. The distribution into the flow eddies is effected directly at and behind the mixer plate and, due to the increased turbulence in the flue gas flow, downstream of the mixer plate. With a large duct cross-section, metering lances are disposed next to one another to fill the cross-section, as well as a plurality of flow plates associated with the metering lances, are provided.
Also proposed has been an introduction of ammonium hydroxide (liquid NH4OH) or urea solution without pre-vaporization directly into the flue gas stream on the back side of a mixer element, whereby the nozzle is arranged on the back side (lee side) of the mixer element in such a way that the direction of introduction extends parallel to the gas stream of the flue gas. The nozzle stream is composed of a mixture of gas and liquid droplets that after a certain period of time are volatilized in the warm environment, which is at approximately 300° C. In this connection there is the danger that reducing agent droplets, together with dust particles contained in the flue gases, lead to concrete-like deposit formation on the mixer element or elements that are used, on support elements for the mixer elements, and possibly on the walls of the flue gas duct. The nozzles must therefore be disposed far enough from the mixer elements that non volatized droplets cannot strike the mixer elements, even under the influence of backflows (eddy trains). This leads to a lengthening of the flue gas duct length that is free of installed components, and which is necessary for the thorough mixing. Additional static mixer elements cannot be installed in the flue gas stream downstream of the metering-in due to the danger of the formation of deposits.
It is an object of the present invention to improve the apparatus of the aforementioned general type in such a way that with a direct metering-in of a liquid as the fluid, in particular liquid reducing agent, with a short mixing path a formation of deposits is prevented to a substantial extent.